A considerable portion of worldwide research efforts in the treatment of cancer is currently devoted to killing cancer cells by means of various cell-killing agents. Despite the fact that numerous drugs, including radioactive compounds, have been shown to be capable of killing cancer cells, these agents frequently fail to treat cancer successfully because of their inability to circumvent three universally present obstacles: (1) the agents do not kill all the cancer cells because they do not exhibit cytotoxic specificity for all the cancer cells, (2) the agents also kill normal cells because they do not exhibit cytotoxic specificity exclusively for cancer cells, and (3) the agents are not potent enough at tolerable doses to kill resistant cancer cells or to overcome the ability of cancer cells to adapt and become resistant to the cell-killing agents.